Cryostat having an effective heat exchanger for cooling its input leads and other leak paths



3,412,320 vs HEAT EXCHANGER FOR COOL INPUT LEADS AND OTHER LEAK PATHSING ITS Nov. 19, 1968 H. 1.. MARSHALL CRYOSTAT HAVING AN EFFECTI FiledMay 16, 1966.

5 Sheets-Sheet 1 l4 mvENTbR. HARRY L. MARSHALL FIG.2

FIG. I

f TTORNEY NOV. 19, 1968 LL 3,412,320

CRYOSTAT HAVING AN EFFECTIVE HEAT EXCHANGER FOR COOLING ITS INPUT LEADSAND OTHER LEAK PATHS Filed May 16, 1966 5 Sheets-Sheet 3 h +-H n [-50 l3 TRANSMITTER PROBE fc um MIXER f0 4, AMPLIFIER v 41 f... 5| FIG. 5

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HARRY L. MARSHALL BY v LJL 1044 TTORNEY Nov. 19, 1968 CRYOSTAT HAVING ANE Filed May 16, 1966 L. MA

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5 Sheets-Sheet 5 POWER SUPPLY v I g 1 I r I f x CURRENT I I 37 SOURCECURRENT SOURCE HARRY L. MARSHALL ORNEY United States Patent 3,412,320CRYOSTAT HAVING AN EFFECTIVE HEAT EXCHANGER FOR COOLING ITS INPUT LEADSAND OTHER LEAK PATHS Harry L. Marshall, Palo Alto, Calif., assignor toVarian Associates, Palo Alto, Calif, a corporation of California FiledMay 16, 1966, Ser. No. 550,382 Claims. (Cl. 324--.5)

The present invention relates in general to cryostats and, moreparticularly, to an improved cryostat including the provision of aneffective heat exchanger using the evolved coolant for removing heatflowing into the cryostat from electrical leads, along Dewar walls andalong the access port cross sectional area to devices within thecryostat. Such an improved cryostat is especially desirable for, but notlimited to, superconductive magnets used with gyromagnetic resonancespectrometers as it substantially decreases the usage of liquid heliumcoolant, and thus reduces operating costs and prolongs the unattendedoperating time.

Heretofore, cryostat systems have been built wherein a portion of theevolved helium coolant gas has been channeled along helical ribbonshaped current leads, running to the device held at cryogenictemperature, by means'of a dielectric tube surrounding the leads. Othershave employed low thermal conductivity plugs closing off the centralhelium chamber to reduce thermal conductivity along the access portcross section leading into the liquid helium chamber. While thesedevices operated to reduce liquid helium consumption they were not veryeffective inasmuch as the temperature of the exhausted coolant gas aswell below the ambient temperature.

In the present invention, the numerous current leads to thesuperconductive solenoid or other device immersed in a cryostat havebeen made of thin sheet metal to provide increased surface area toenhance heat transfer to the exhaust coolant gas.

These thin leads have been positioned along the outside of the thermallyinsulative plug for optimizing heat transfer to the annular exhaust gascolumn. Moreover, the

width of the annular gas column has been reduced to increase thevelocity of the exhaust coolant to a regime of turbulent flow to enhanceheat transfer. In a preferred embodiment, which for most of itsoperating time requires very little current to flow in the leads, theleads have been made of an alloy, such as brass, which has an integratedratio of electrical conductivity to thermal conductivity, over thecryogenic temperature range of interest, which is substantially greaterthan such ratio for copper, thereby substantially reducing the thermalconduction heat leak down the current leads in their non energizedstate. By employing the above described improvements the liquid coolantusage rate has been reduced by a factor of 2.5.

The principal object of the present invention is the provision of animproved cryostat.

One feature of the present invention is the provision of current leadsto a device held at cryogenic temperatures and contained in a cryostatwherein the leads are made of thin metal and are carried about theperiphery of a thermally insulative plug partially closing off a liquidcoolant chamber of the cryostat, whereby the leads are placed in heatexchanging relationship to the exhaust coolant gas.

Another feature of the present invention is the same as the precedingfeature wherein the plug is dimensioned relative to the inside Wall ofthe liquid coolant chamber to reduce the width of the annular exhaustcolumn to a value which increases the exhaust gas velocity to aturbulent flow regime, whereby heat transfer to the exhaust gas isincreased.

Another feature of the present invention is the same as of ribbon shape,are disposed about the periphery of the 3,412,320 Patented Nov. 19, 1968ice any one or more of the preceding wherein the thin leads are made ofan alloy having a ratio of electrical conductivity to thermalconductivity, as integrated over the cryogenic range of interest, whichis greater than such ratio for copper.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

FIG. 1 is a side elevational view of a magnet system employing featuresof the present invention,

FIG. 2 is an enlarged longitudinal sectional view of a portion of thestructure of FIG. 1 taken along line 2-2 in the direction of the arrows,

FIG. 3 is a sectional view of the structure of FIG. 2 taken along line33 in the direction of the arrows,

FIG. 3a is an enlarged fragmentary view of a portion of the structure ofFIG. 3 delineated by line 3A3A,

FIG. 4 is a schematic circuit diagram for the magnet of FIG. 1, and

FIG. 5 is a block circuit diagram for a gyromagnetic resonancespectrometer using the magnet system of FIG. 1.

Referring now to FIG. 1 there is shown a superconductive magnet systememploying features of the present invention. The system includes asuperconductive solenoid, immersed in a liquid helium bath containedwithin a cylindrical cryostat 2. The cryostat 2 with its internalsolenoid is held in the vertical position by a magnet stand 3. Themagnet stand 3 includes a centrally apertured magnet carriage member 4holding the cryostat by means of a support flange 5 welded to themidsection of the cryostat 2. The magnet carriage is supported from avertical shaft 6 upstanding from a base plate 7. A probe carriageassembly 3 is axially movable along the shaft 6 and includes ahorizontal support plate 9 which holds a field utilization probe 11. Theprobe includes a vertical neck portion 12 containing the sample to beimmersed in the strong field of the magnet 1. The neck portion 12 isinserted from the bottom of the cryostat 2 into an axially re-entrantbore 13 in the cryostat 2. The magnet stand 3 forms the subject matterof and is claimed in copending US. application 544,775, filed Apr. '25,1966, and assigned to the same assignee as the present invention.

Referring now to FIGS. 2 and 3 there is shown in more detail magnet 1with its cryostat 2. The superconductive solenoid magnet 1 is coaxiallydisposed of the re-entrant bore portion 13 of the cryostat 2 in acentral liquid helium cryogenic fluid chamber 14. A cylindrical wall 15of the helium chamber 14 is made of thin stainless steel tubing as of0.020 thick sheet metal with an inside diameter of 8.250 and a length ofabout 48". A thin cylindrical vacuum chamber 16 envelops the liquidhelium chamber 14. A liquid nitrogen chamber 17 envelops the vacuumchamber 16 and an outer vacuum chamber 18 envelops the liquid nitrogenchamber 17. The outer wall of the vacuum chamber 18 forms the outer wallof the cryostat 2. The cryogenic fluid and vacuum chambers 14-18 form aDewar portion of the cryostat 2.

A thermally insulative cylindrical plug 19 partially closes off theupper end of the cylindrical central liquid helium chamber 14. The plug19 is, for example, 14 long and 8.125" in diameter and includes threedisk shaped transverse headers 21 as of 7 thick epoxy glass. The headers21 are covered with a thin sheet of epoxy glass as of ,4 thick sheet,thereby forming the cylindrical side Walls 22 of the plug 19. Theinterior of the plug 19 is filled with a thermally insulative foammaterial 23 such as polyurethane foam or a glass foam.

A plurality of thin metal electrical leads 24, preferably plug 19. Asused herein thin means that the conductor has a depth transverse to thedirection of current transport, which is less than /2 of its othertransverse extent. It is intended to encompass many differentconfigurations including, ribbon shape, helical ribbon shape, andvarious thin walled hollow tubular shapes. In one embodiment of thepresent invention the leads 24 are ribbon shaped. Certain ones of theleads 24 carry the 15-20 amp energizing current to the magnet 1 and are,for example, metal strips 15" long, 0.010" deep and A wide. Other onesof the leads 24 carry less current and are, for example, long, 0.002" or0.005 deep and wide.

The ribbon leads 24 are laid fiat against the exterior 22 of the plug 19with their exposed flat sides facing the thin annular passageway 25, asof 0.063" in radial thickness, defined between the exterior wall 22 ofthe plug 19 and the adjacent cylindrical chamber wall 26 of the liquidhelium chamber 14. Dielectric strips having side edge lip portions whichoverlay the edges of the ribbon leads 24 hold the leads against the plug19.

The helium chamber 14 is filled with liquid helium to a liquid level 27above the solenoid 1, and preferably up to the plug 19. Thus the plugextends from ambient to liquid helium temperatures. In a typicalexample, this requires about 15 liters of liquid helium. As the liquidhelium absorbs heat, due to heat leaks into the cryostat 2, helium gasis evolved. Typical heat leaks include thermal conduction paths down theleads 24, down the wall 26, and down the plug 19. The evolved helium gaSis exhausted from the helium chamber 14 via the narrow annularpassageway 25.

The radial thickness of the annular passageway 25 is preferably madesufiiciently small such that the exhausting helium gas flow is withinthe turbulent flow regime as contrasted with the lower velocity laminarflow regime. Turbulent flow is characterized by substantially greaterheat transfer from the leads 24 and wall 26 to the annular column ofexhausting helium gas. In the present case, turbulent flow was achievedwhen the radial thickness of the passageway 25 was reduced to less than0.125" and with the 0.063" passageway 25 about 175 cmF/hr. of liquidhelium was being exhausted through the passageway to the atmosphere.

For a cryostat wherein the electrical leads 24 are typically notcarrying current during operation of the system, the leads 24 arepreferably made of a material having an integrated ratio of electricalconductivity to thermal conductivity over the range of cryogenictemperature of interest which is higher than that of copper over therange of cryogenic temperatures of interest. Examples of such materialsare generally metal alloys such as brass, stainless steel, Monel, andmanganin, etc.

In a magnet system as aforedescribed, using brass leads 24, theconsumption of liquid helium was reduced from 500 cmfi/hr. to 175cmP/hr. as compared to a system wherein the leads 24 passed through apair of axially spaced radiation shields closing off the central liquidhelium chamber.

The plug 19 and solenoid 1 are carried on four longitudinally directedthin walled stainless steel tubes 29 connected at their upper ends to analuminum cap 31 and at their lower ends to the solenoid 1. A radiationshield 32, as of thin copper sheet, is transversely carried on the tubes29 inbetween the plug 19 and solenoid 1 for shielding the liquid heliumfrom heat radiating down the chamber 14. Terminal lugs 33 are providedat the upper and lower ends of the thin leads 24. The lower lugs 33connect to the solenoid via conventional copper leads or superconductingwire. The upper lugs 33 connect to various power supplies and protectivediode circuits as more fully described below via thermally insulatedcopper wire.

Referring now to FIG. 4 there is shown the electrical circuit for thesuperconductive magnet 1. The magnet includes the solenoid winding 1formed, for example, by

120,000 feet of superconductive NbZr wire wound into a solenoid having abore of 1.8", and a length of about 16". A current regulated constantcurrent source 35 is connected across the end terminals of the solenoid1 for energizing the solenoid with about 15-20 amps of current toproduce a D.C. magnetic field of about 60 kg. The solenoid 1 issegmented into several sections which are tapped out via leads to a bankof series connected diodes 36 comprising two strings of diodes with onestringof diodes connected to pass current in each of two directions.across the solenoid 1 and each segment of the solenoid. This diode bank36 protects the solenoid and current sources against excesive voltagesbeing developed in the circuit and forms the subject matter of and isclaimed in copending US. application 543,666, filed Apr. 19, 1966, andassigned to the same assignee .as the present invention.

The various segments of the solenoid winding 1 are divided into threegroups, a large central group and two smaller end groups. Separatecurrent regulated constant current sources 37 .are connected acrossthese groups of windings for permitting separate adjustment of thecurrent in the three portions of the solenoid winding 1. Separatecontrol of these currents permits cancellation of certain axialgradients in the magnetic field produced by the solenoid 1. The separatecurrent sources form the subject matter of and are claimed in copendingUS. application 548,009, filed May 5, 1966, and assigned to the sameassignee as the present invention.

Superconductive wires 38 are connected across the solenoid 1 in parallelwith the respective current sources 35 and 37 for permitting thecirculating current of the solenoid 1 to be shifted from the circuitloop portions which include the current sources to the respectiveparallel loop portions which include the superconducting wires 38. Thisis accomplished by heating the wires 38 via heaters 30 during currentenergization of the solenoid 1. After the magnet is fully energized withthe various loop currents adjusted for optimum field homogeneity, theheaters 39 are de-energized and when the wires 38 become superconductivethe current is shifted from the power supplies 35 and 37 to the wires 38by decreasing the current supplied from the respective sources. When thesources 35 and 37 have transferred their currents to the lWlI8S 38 theyare disconnected from the solenoid at points external of the diode bank36. The heaters 39 are energized from a power supply 41 and leadsconnected across a voltage divider network 42.

Certain transverse field gradients are cancelled by means of separatelycurrent adjustable coil sets 43 mounted on the outside of thesolenoid 1. The current is supplied to these coil sets 43 from agrounded centertapped battery 44 via two potentiometers 45 connectedacross the battery 44. The various current leads 24, as shown in thediagram of FIG. 4 in the region where they pass inside the liquid heliumchamber 14, are made of the brass ribbon configuration to enhance heattransfer therefrom and to inhibit heat conduction into the heliumchamber 14.

Referring now to FIG. 5 there is shown the electrical circuitry forobserving the gyromagnetic resonance spectrum of the sample underanalysis. A field modulator 47 superimposes an alternating magneticfield component Hm, at a convenient audio frequency, as of 10 kHz, onthe D.C. field H over the sample volume within the probe 11. An ultrahigh frequency transmitter 48 applies an alternating magnetic field H tothe sample at a frequency f which is displaced in frequency from thegyromagnetic resonance frequency f of the sample by the field modulationfrequency fm. The U.H.F. magnetic field H is polarized at right anglesto the D.C. field. Under these conditions, gyromagnetic resonance of thesample is excited at f,,, which may be on the order of 220 mHz. Theexcited resonance is frequency modulated having a carrier resonancecomponent at f and Bessel function amplitude sidebands at frequencyintervals separated in frequency by the field modulation frequency fm.The fm resonance signal emanating from the sample is picked up in areceiver coil, located within the probe 11, and fed to U.H.F. amplifier'50 and thence to a mixer 49. In the mixer, the resonance signal ismixed with a sample of the transmitter signal to obtain anaudio-frequency resonance signal at the field modulation frequency fm.The resonance signal is then amplified by audio amplifier 51 and fed toone input of a phase sensitive detector 52 wherein it is compared with asample of the field modulation signal to obtain a D.C. resonance outputsignal. The D.C. polarizing magnetic field H is scanned through theresonance spectrum of the sample under analysis by superimposing a scanfield component Hs obtained from a scan generator 53, upon thepolarizing field H within the sample volume. The D.C. output resonancesignal from the phase sensitive detector 52 is fed to a recorder 54 forrecording as a function of time or scan field intensity as obtained fromthe scan generator 53.

Although the superconductive magnet system of the present invention hasbeen explained as it would be used in conjunction with a gyromagneticresonance spectrometer, it may be used with other types of fieldutilization devices wherein a sample is inserted into an intensemagnetic field.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A cryostat apparatus including, means forming a device to be held atcryogenic temperatures, means forming a cryostat enveloping said devicemeans and having a chamber for containing a cryogenic fluid for coolingsaid device means to its cryogenic temperature, means forming aplurality of electrical leads passing into said cryostat and connectingto said device, said leads being made of thin conductor, means forming aplug partially closing off said chamber and defining a gas passageway inthe space between said plug and the inside wall of said chamber andthrough which evolved gaseous cryogenic fluid is exhausted from saidchamber for cooling the inside wall of said chamber, and wherein saidleads are arranged about the periphery of said plug adjacent said gaspassageway in heat exchanging relation to said exhaust gas for coolingof said leads.

2. The apparatus of claim '1 wherein said thin conductor leads areribbon shaped.

3. The apparatus of claim 1 wherein said plug is elongated and made of athermally insulative material, and wherein said leads are ribbon shapedand laid flat against the outside periphery of said plug.

4. The apparatus of claim 1 wherein said leads are made of an alloymaterial having a ratio of electrical conductivity to thermalconductivity which is higher than that ratio for copper at the cryogenictemperatures of said thin leads.

5. The apparatus of claim 1 wherein said plug is dimensioned relative tothe inside wall of said chamber in the region of said leads to reducethe dimensions of the gas passageway to the extent such that thevelocity of the exhausting gas in the region adjacent said leads fallWithin the turbulent flow regime to enhance heat transfer from saidleads to the exhaust gas.

6. The apparatus of claim 5 wherein said plug is an elongated thermallyinsulative cylindrical structure, and said passageway is an arcuatecolumn having a radial thickness of less than 0.100".

7. The apparatus of claim 6 wherein said leads are made of brass.

8. The apparatus of claim 1 wherein said device means is asuperconductive solenoid magnet.

9. The apparatus of claim 8 including in combination, means forimmersing a sample of gyromagnetic resonance substance under analysis inthe magnetic field of said solenoid, means for producing gyromagneticresonance of the sample in the magnetic field, and means for detectingthe gyromagnetic resonance of the sample to obtain a gyromagneticresonance output.

10. The apparatus of claim 1 wherein the cryogenic fluid is a liquid,wherein said plug is elongated and made of a thermally insulativematerial, and wherein said plug has a longitudinal extent which extendsfrom substantially ambient temperature to substantially the temperatureof the cryogenic liquid in said chamber which is closed off by saidplug.

References Cited UNITED STATES PATENTS 3,080,527 3/1963 Chester 33043,133,144 5/1964 Cottin-gham 335216 3,286,014 11/1966 Williams 335-4163,349,161 10/ 1967 Latham l74--15 RUDOLPH V. ROLINEC, Primary Examiner.

M. J. LYNCH, Assistant Examiner.

1. A CRYOSTAT APPARATUS INCLUDING, MEANS FORMING A DEVICE TO BE HELD ATCRYOGENIC TEMPERATURES, MEANS FORMING A CRYOSTAT ENVELOPING SAID DEVICEMEAN AND HAVING A CHAMBER FOR CONTAINING A CRYOGENIC FLUID FOR COOLINGSAID DEVICE MEANS TO ITS CRYOGENIC TEMPERATURE, MEANS FORMING APLURALITY OF ELECTRICAL LEADS PASSING INTO SAID CRYOSTAT AND CONNECTINGTO SAID DEVICE, SAID LEADS BEING MADE OF THIN CONDUCTOR, MEANS FORMING APLUG PARTIALLY CLOSING OFF SAID CHAMBER AND DEFINING A GAS PASSAGEWAY INTHE SPACE BETWEEN SAID PLUG AND THE INSIDE WALL OF SAID CHAMBER ANDTHROUGH WHICH EVOLVED GASEOUS CRYOGENIC FLUID IS EXHAUSTED FROM SAIDCHAMBER FOR COOLING THE INSIDE WALL OF SAID CHAMBER, AND WHEREIN SAIDLEADS ARE ARRANGED ABOUT THE PERIPHERY OF SAID PLUG ADJACENT SAID GASPASSAGEWAY IN HEAT EXCHANGING RELATION TO SAID EXHAUST GAS FOR COOLINGOF SAID LEADS.